Method and apparatus for multi-color printing using hybrid dot-line halftone composite screens

ABSTRACT

A method and apparatus for generating a multicolor image using halftone screens employs a dot structure dot growth pattern for one or more of the colors and a line structure dot growth pattern for at least two or more of the other colors.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional application of application Ser. No. 10/837,518filed Apr. 30, 2004. This application is related to U.S. applicationSer. No. 10/836,762 filed Apr. 30, 2004 in the names of Tai et al. andentitled, METHOD AND APPARATUS FOR MULTI-COLOR PRINTING USING A ROSETTEOR DIAMOND HALFTONE SCREEN FOR ONE OR MORE OF THE COLORS, the contentsof which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of digital encoding ofpictorial information for use in forming color reproductions on displayor printing systems.

2. Description Relative to the Prior Art

With the advent of printing using digital technology, images may beprinted, by rendering the image into a set of pixels. In pure binaryprinters, the pixel is either on (black) or off (white). Such techniquesare well suited to reproducing text because the sizes of the individualpixels that make up the symbols are much smaller than the symbols. Thus,the human eye sees the text as a continuous image even though it is acollection of closely spaced dots.

However, most binary print engines and particularly electrophotographicprint engines do not provide acceptable levels of gray for other images,such as photographs. Those skilled in the art have used halftone dots toemulate grayscale for reproducing images with continuous tones. Onereason for this is that the particles used for forming the printed dotsmay be larger than is desirable even if the printing system were suitedto printing very small binary pixels.

In the area of digital printing (the term “printing” is used toencompass both printing and displaying throughout), gray level has beenachieved in a number of different ways. The representation of theintensity, i.e., the gray level, of a color by binary displays andprinters has been the object of a variety of algorithms. Binary displaysand printers are capable of making a mark, usually in the form of a dot,of a given, uniform size and at a specified resolution in marks per unitlength, typically dots per inch. It has been well known to place themarks according to a variety of geometrical patterns such that a groupof marks when seen by the eye give a rendition of an intermediate colortone between the color of the background (usually white paper stock) andtotal coverage, or solid density. The effect is such that a group ofdots and dot-less blank spots, when seen by the eye, is a rendition ofan intermediate color tone or density between the color of the initialpaper stock, usually white, and total ink coverage, or solid densityhalftone dot. It is conventional to arrange the dots in rows, where thedistance between rows is known as line spacing, and determines thenumber of lines per inch (lpi). In the ensuing paragraphs, discussionswill be made in terms of white paper stock; it is understood that whitepaper stock is used as an illustration and not as a limitation of theinvention and that other media may be used such as plastics, textiles,coated papers, metals, wood, edible articles, etc.

Continuous tone images contain an apparent continuum of gray levels.Some scenes, when viewed by humans, may require more than two hundredand fifty six discrete gray levels for each color to give the appearanceof a continuum of gray levels from one shade to another. Halftonepictorial or graphical images lower the high contrast between the paperstock and toned image and thereby create a more visually pleasing image.As an approximation to continuous tone images, pictorial imagery hasbeen represented via binary halftone technologies. In order to record ordisplay halftone images one picture element of the recording or displaysurface consists of a j×k matrix or cell of sub-elements where j and kare positive integers. A halftone image is reproduced by printing therespective sub-elements (pixels or pels) or leaving them blank, in otherwords, by suitably distributing the printed marks within each cell.

Another method of producing gray levels, is provided by gray levelprinting. In such a method, each pixel has the capability to renderseveral different dot sizes. In certain electrophotographic printingsystems, for example, the dot size for a pixel is a function of theexposure time provided an LED element corresponding to that pixel. Thelonger the exposure time, the more toner is attracted to that particularpixel.

There are two major concerns in rendering a continuous tone image forprinting: (1) the resolution of image details, and (2) the reproductionof gray scales. These two fundamental factors compete with each other ina binary representation scheme. The more gray levels that are rendered,the larger is a halftone cell. Consequently, coarse halftone linescreens are provided, with the attendant poor image appearance. Hence,compromises made in rendering between the selection of line resolutionin gray scale and binary halftone printing. However, with gray levelhalftone printing, one can satisfy both resolution and gray levelrequirements. In gray level printing, the same number of addressabledots are present, and there is attached a choice of dot sizes from onedot size of 1 bit/pixel to for example 255 different dot-sizes of 8bits/pixel. Although providing higher image quality with respect to lineresolution and tone scales, gray level halftone presents its own dotrendering issues.

A number of different dot layouts are possible to build gray level dotsfrom a cell template. These gray level dots are the digitalrepresentation of the gray level screening, and must be realized througha printing process. It is desirable in gray level screening to layoutthe dots with the printing process characteristics built into it suchthat the appearance of the dots are pleasing to the eye: less grainy,stable, less artifacts, less texture (i.e., visible screen and itsmicrostructure).

An example of a line screen designed for gray scale rendering isdisclosed in U.S. Pat. No. 5,258,850. The arrangement of pixels within ahalftone cell is such that growth within a cell to represent increasesin density is accomplished through arranging the pixels along lines ofgrowth. Another example of a halftone cell is that shown in U.S. Pat.No. 5,258,849, which features growth of density within a halftone cellby gradual enlargement about a central area within the cell. Thehalftone cells disclosed in the above two patents are notable in thatthe pixels we need within each cell may vary in density. Thissubstantially increases the number of gray levels that may berepresented by the overall halftone cell from that where the pixels canonly be rendered as a binary representation (either black or white withno distinction regarding size). The combination of cells represents ahalftone screen.

Color printing on halftone printers involves the formation of colorseparations as halftone screens for each color, which is to be used toform a color image. The halftone screens are laid down on apredetermined overlapping relationship to each other, which results ingeneration of the desired color image. A well-known problem whenoverlapping two or more halftone screens is the possibility ofdeveloping a moiré pattern or other form of interference, when thescreens are not properly positioned. To avoid the moiré or otherundesirable patterns, precise angle combinations of the screens arerequired. It is known that increasing the difference in angle of twooverlaid dot screens will result in a smaller pattern, making thepattern less apparent. However, the prior art teaches, see for exampleU.S. Pat. No. 6,307,645, the largest possible angle difference betweentwo overlaid screens should be no more than 45° because a 90° screen isessentially the same as 0°, just as a 135° screen is the same as a 45°screen even in the context of attempting to reduce moiré withasymmetrical dots.

In color image printing it has been common practice to use at leastthree process colors and in more cases three process colors and black.In the case of four-color printing the printing industry has generated astandardized combination of four halftone angles. In particular and withreference to FIG. 1, the cyan halftone screen is located at 15°, theblack halftone screen at 45°, the magenta halftone screen at 75° and theyellow halftone screen at 0°. Since yellow is the lightest and leastnoticeable color, it can be set at 0°, even though 0° is a highlynoticeable angle, and that is only 15° from the nearest neighbor. Insome embodiments, the cyan halftone screen is known to be set at 105°,however, with symmetrical dots this is substantially the same as 15°,and the prior art recognizes that even with asymmetrical dots it doesnot make a large difference.

When the four process colors using the above halftone screen anglecombinations are overlaid, the resulting moiré or other interferencepatterns are as small as possible. A visually pleasing rosette structureis formed when the individual dots grains are oriented 30° apart. Thetraditional graphics art printing has been made using this 15°/45°/75°angle screen design to form a balanced rosette structure. In the CMYKfour-color printing process, the yellow screen is usually designed at 0or 45°. However, the moiré pattern resulting from the interactionbetween the yellow screen and the other three individual screens due tomiss-registration is not as visually pleasing as a 30° moiré pattern(rosette structure). Yellow is a light color, so this additional moiréis usually acceptable and not very noticeable in most CMYK four-colorprinting systems. However, careful examination of prints shows that thisyellow moiré pattern can be seen in certain composite colors. Whereadditional colors are used such as in a hi-fi color (for example, afive-color) printing system, there is a need to design a fifth screen ontop of the original well-balanced CMYK screen set. This is particularlytrue where the fifth color screen is blue, the complementary color ofyellow, and the blue color screen is placed at the same screen angle andscreen frequency as the yellow color screen. The unpleasant moiré, whichwas not noticeable in the yellow color, will now show up in the bluecolor.

It is thus known that many color printing systems will include five ormore printing units using different color colorants. Attempting toincorporate these additional colors is noted to be difficult, especiallyif each color must have a halftone screen with a unique halftone angle.Particularly, once there are more than four screens with attendantscreen angles, which must be laid down, the patterning problemsdiscussed above, are greatly increased. It would thus be desirable toprovide color screen sets for printing which minimize the unpleasantmoiré patterns formed including those caused by the interactions of theyellow screen.

SUMMARY OF THE INVENTION

The foregoing objects are realized by the present invention, whichprovides an apparatus and method for the generation of halftone imageswith reduced image artifacts and increased number of gray levels. Inaccordance with a first aspect of the invention there is provided anapparatus for configuring image information for printing of an imagehaving at least three different colors, comprising a first halftonescreen generator for generating information representing a first colorseparation image of the image by generating first halftone cells at afirst predetermined halftone screen angle wherein pixels within thefirst halftone cell are oriented in a line structure with increasingcell density from first halftone cell to first halftone cell beingidentified by stronger line structures; a second halftone screengenerator for generating information representing a second colorseparation image of the image by generating second halftone cells at asecond predetermined halftone screen angle different from the firsthalftone screen angle and wherein pixels within the second halftone cellare represented in a line structure with increasing cell density fromsecond halftone cell to second halftone cell being identified bystronger line structures, the line structures formed in the secondhalftone cells being at a different angle than the line structuresformed in the first halftone cells; and a third halftone screengenerator for generating information representing a third colorseparation image of the image by generating third halftone cells at athird predetermined halftone screen angle wherein pixels within a thirdhalftone cell are represented by dots with increasing cell density fromthird halftone cell to third halftone cell being identified by strongerdot structures, the dot structures of the third halftone cells imagebeing ordered along a series of parallel third lines and the series ofparallel third lines being at a different angle than that of the linestructures formed in the first and second halftone cells.

In accordance with a second aspect of the invention there is provided amethod for configuring image information for printing of an image havingat least three different colors, comprising generating informationrepresenting a first color separation image of the image by generatingfirst halftone cells at a first predetermined halftone screen anglewherein pixels within the first halftone cell are oriented in a linestructure with increasing cell density from first halftone cell to firsthalftone cell being identified by stronger line structures; generatinginformation representing a second color separation image of the image bygenerating second halftone cells at a second predetermined halftonescreen angle different from the first halftone screen angle and whereinpixels within the second halftone cell are represented in a linestructure with increasing cell density from second halftone cell tosecond halftone cell being identified by stronger line structures, theline structures formed in the second halftone cells being at a differentangle than the line structures formed in the first halftone cells; andgenerating information representing a third color separation image ofthe image by generating third halftone cells at a third predeterminedhalftone screen angle wherein pixels within a third halftone cell arerepresented by dots in accordance with a dot structure growth patternwith increasing cell density from third halftone cell to third halftonecell being identified by stronger dot structures, the dot structures ofthe third halftone cells image being ordered along a series of parallelthird lines and the series of parallel third lines being at a differentangle than that of the line structures formed in the first and secondhalftone cells.

In accordance with a third aspect of the invention there is providedapparatus for configuring image information for printing of an imagehaving at least three different colors, comprising a screen generator orgenerators for generating halftone screen color separation image datafor each of the at least three different colors, the screen generator orgenerators generating image data in the form of halftone screensrepresentative of the image at predetermined respective screen angleswherein for each of at least two of the three colors a first type of dotgrowth is pattern is provided and is operational to generate halftonecells of different density and for the third of the three colors asecond type of dot growth pattern is provided which is different fromthe first type of dot growth pattern used commonly for the said at leasttwo of the three colors, wherein the first type of growth pattern isselected from the group consisting of the dot structure dot growthpattern and the line structure dot growth pattern and the second type ofgrowth pattern is selected from the group consisting of the dotstructure dot growth pattern and the line structure dot growth patternand further wherein the respective screen angles for the at least two ofthe three colors and for the third of the three colors are all differentfrom each other.

In accordance with a fourth aspect of the invention there is provided amethod for configuring image information for printing of an image havingat least three different colors, comprising generating halftone screencolor separation image data for each of the at least three differentcolors in the form of halftone screens representative of the image atpredetermined respective screen angles wherein for each of at least twoof the three colors a first type of dot growth is pattern is providedand is operational to generate halftone cells of different density andfor the third of the three colors a second type of dot growth pattern isprovided which is different from the first type of dot growth patternused commonly for the said at least two of the three colors, wherein thefirst type of growth pattern is selected from the group consisting ofthe dot structure dot growth pattern and the line structure dot growthpattern and the second type of growth pattern is selected from the groupconsisting of the dot structure dot growth pattern and the linestructure dot growth pattern and further wherein the respective screenangles for the at least two of the three colors and for the third of thethree colors are all different from each other.

In accordance with a fifth aspect of the invention there is provided amethod for generating a multicolor image using halftone screenscomprising forming halftone dots using a dot structure dot growthpattern for the halftone cells of one or more of the colors; and forminghalftone lines using a line structure dot growth pattern for thehalftone cells of at least two of the other colors.

Other objects, advantages, and novel features of the present inventionwill become more apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a representation of halftone screenangles in a four-color printing system as known in the prior art;

FIG. 2 is a schematic of an electrophotographic print engine that may beused in accordance with the invention to generate multicolor prints;

FIG. 3 is a diagram illustrating a representation of halftone screenangles and dot growth pattern types (dot or line) in a five colorprinting system in accordance with a first embodiment of the invention;

FIG. 4 illustrates an exemplary three-bit gray halftone dot layoutsaccording to a full dot type embodiment as known in the prior art andwhich may be used in accordance with the invention;

FIG. 5 illustrates a halftone cell with dots that have been formed inaccordance with the full dot type of growth pattern of FIG. 4;

FIG. 6 illustrates an exemplary halftone dot mask used for growing thefull dot type dot of FIG. 4;

FIG. 7 is a graphic illustration of the building up within a halftonecell in accordance with a line structure type dot growth pattern asknown in the prior art and which may be used in accordance with theinvention;

FIG. 8 is a diagram illustrating a representation of halftone screenangles and dot growth pattern types (dot or line) in a five colorprinting system in accordance with a second embodiment of the invention;

FIG. 9 is a diagram illustrating a representation of halftone screenangles and dot growth pattern types (dot or line) in a four colorprinting system in accordance with a third embodiment of the invention;

FIG. 10 is an illustration of a diamond screen structure formed by twocolor separation images each having a line structure dot growth patternand wherein the line structures are at an angle of 60° to each other;

FIG. 11 is an illustration of a density ramp of the two color separationimages that feature the diamond structure of FIG. 10;

FIGS. 12A and 12B are diagrams each illustrating a representation ofhalftone screen angles and dot growth pattern types (dot or line) in afive color printing system in accordance with fourth and fifthembodiments of the invention;

FIG. 13 is a diagram illustrating a representation of halftone screenangles and dot growth pattern types (dot or line) in a five colorprinting system in accordance with a six embodiment of the invention;

FIGS. 14 and 15 are diagrams each illustrating a representation ofhalftone screen angles and dot growth pattern types (dot or line) in afive color printing system in accordance with seventh and eighthembodiments of the invention;

FIG. 16 is a flowchart of a process for creating a single hi-fi colorseparation image in a five color printing system of FIGS. 14 and 15; and

FIGS. 17 and 18 are graphs illustrating weighting factors that might beused in the process of FIG. 16.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 is an elevational view showing the essential portions of anelectrophotographic engine suitable for printing of full-color imagesand incorporating the improvements of the invention. Although oneembodiment of the invention involves printing using anelectrophotographic engine having repeating sets of single color imageproducing stations and arranged in a so-called tandem arrangement otherelectrostatographic color reproduction apparatus can make use of theinvention as well as other types of color printing systems includinginkjet, lithography, etc.

With reference now to FIG. 2 there is shown a printer apparatus 500having a number of tandemly arranged electrostatographic image formingmodules. Although five modules are shown it will be understood that theinvention is applicable to a printer apparatus for printing at leastthree more colors. Each module of the printer includes a plurality ofelectrophotographic imaging subsystems for producing a single colortoned image. Included in each imaging subsystem is a charging subsystemfor charging a photoconductive imaging member, an exposure system forimagewise exposing a photoconductive imaging member to form a latentcolor separation image in the respective color, a development subsystemfor toning the imagewise exposed photoconductive imaging member withtoner of the respective color, an intermediate transfer subsystem fortransferring the respective color separation image from thephotoconductive imaging member to an intermediate transfer member andfrom the intermediate transfer member to a receiver member whichreceives the respective toned color separation images in superpositionto form a composite multicolor image. Subsequent to transfer of therespective color separation images from each of the respectivesubsystems the receiver member is transferred to a fusing subsystem tofuse the multicolor toner image to the receiver member. Further detailsregarding the printer 500 are also provided in U.S. Pat. No. 6,608,641,the contents of which are incorporated herein by reference.

The five exemplary color modules of printer apparatus 500 are forpreferably forming black, cyan, magenta, yellow, and blue color tonerseparation images. Although blue is illustrated and preferred as thefifth color it will be understood that the fifth color may be otherdominant colors such as red or green or orange or violet or that thenumber of the modules may be increased to print more colors than five.Elements in FIG. 2 that are similar from module to module have similarreference numerals with a suffix of B, C, M, Y, and BE referring to acolor module to which it is respectively associated; i.e. black (B),cyan (C), magenta (M), yellow (Y), and blue (BE). Each module (591B,591C, 591M, 591Y, and 591BE) is of similar construction except that asshown one receiver transport web (RTW) 516 in the form of an endlessbelt operates with all the modules and the receiver member istransported by the RTW 516 from module to module. Receiver members aresupplied from a paper supply unit, thereafter preferably passing througha paper conditioning unit (not shown) before entering the first modulein the direction as indicated by arrow A. The receiver members areadhered to RTW 516 during passage through the modules, eitherelectrostatically or by mechanical devices such as grippers, as is wellknown. Preferably, receiver members are electrostatically adhered to RTW516 by depositing electrostatic charges from a charging device, such asfor example by using a tack-down corona charger 526. Five receivermembers or sheets 512 a, 512 b, 512 c, 512 d, and 512 e are shown(simultaneously) receiving images from modules 591BE, 591B, 591C, 591M,and 591Y. It will be understood as noted above that each receiver membermay receive one color image from each module and that in this example upto five-color images can be received by each receiver member. Themovements of the receiver member with the RTW 516 is such that eachcolor image transferred to the receiver member at the transfer nip 510B,510C, 510M, 510Y, and 510BE of each module is a transfer that isregistered with the previous color transfer so that a five-color imageformed on the receiver member has the colors in registered superposedrelationship on the transferee surface of the receiver member. Thereceiver members are then serially detacked from RTW 516 and sent in adirection indicated by arrow B to a fusing station (not shown) to fuseor fix the dry toner images to the receiver member. The RTW isreconditioned for reuse by providing charge to both surfaces using, forexample, opposed corona chargers 522, 523 which neutralize charge on thetwo surfaces of the RTW.

Each color module includes a primary image-forming member, for example adrum or primary image-forming roller (PIFR) labeled 503B, 503C, 503M,503Y, and 503BE respectively. Each PIFR 503B, 503C, 503M, 503Y, and503BE has a respective photoconductive surface structure 507B, 507C,507M, 507Y, and 507BE having one or more layers, upon which a pigmentedmarking particle image or a series of different ones of such images isformed (individual layers of PIFRs are not shown). In order to formtoned images, the outer surface of the PIFR is uniformly charged by aprimary charger such as a corona charging device 505B, 505C, 505M, 505Y,and 505BE respectively, or by other suitable charger such as a rollercharger, a brush charger, etc. The uniformly charged surface ispreferably exposed by a respective electronic image writer, whichexposure device is preferably an LED or other electro-optical exposuredevice, for example, a laser to selectively alter the charge on thesurface of the PIFR. The exposure device creates an electrostatic imagecorresponding to an image to be reproduced or generated. Theelectrostatic image is developed, preferably using the well-knowndischarged area development technique, by application of pigmentedmarking particles to the latent image bearing photoconductive drum bydevelopment station 581B, 581C, 581M, 581Y, and 581BE respectively,which development station preferably employs so-called “SPD” (SmallParticle Development) developers. Each of development stations 581B,581C, 581M, 581Y, and 581BE is respectively electrically biased by asuitable respective voltage to develop the respective latent image,which voltage may be supplied by a power supply, e.g., power supply 552,or by individual power supplies (not illustrated). The respectivedeveloper includes toner marking particles and magnetic carrierparticles. Each development station has a particular color of pigmentedtoner marking particles associated respectively therewith for toning.Thus, each module creates a series of different color marking particleimages on the respective photographic drum. In lieu of a photoconductivedrum, which is preferred, a photoconductive belt may be used.Alternatively, the image may be created by an electrostatic charger thatforms respective pixels of charge on an insulating surface directly inresponse to image information.

Each marking particle image formed on a respective PIFR is transferredto a compliant surface of a respective secondary or intermediate imagetransfer member, for example an intermediate transfer Roller (ITR)labeled 508B, 508C, 508M, 508Y, and 508BE respectively. After transfer,the residual toner image is cleaned from the surface of thephotoconductive drum by a suitable cleaning device 504B, 504C, 504M,504Y, and 504BE, respectively, so as to prepare the surface for reusefor forming subsequent toner images.

A logic and control unit (LCU) provides various control signals thatcontrol movement of the various members and the timing thereof as wellas the appropriate electrical biases for accommodating the varioustransfers of the respective toner images. Timing signals are alsoprovided to a motor, M, which drives a drive roller 513 that drives theRTW 516. The RTW in turn may be used to drive the other componentsand/or other drivers may be used to control movement of the rollers inthe respective modules. Image data for writing by the printer apparatus500 may be processed by a raster image processor (RIP) 501 which mayinclude a color separations screen generator or generators. The term“generator” or “generators” are used interchangeably herein since asingle device may operate serially and be programmed or adjusted tooperate differently for each of the different screens. The output of theRIP may be stored in a frame or line buffers 502 for transmission of thecolor separation print data to each of the respective LED writers,506BE, 506B, 506C, 506M, and 506Y. The RIP and/or color separationsscreen generator may be a part of the printer apparatus or remotetherefrom. Image data processed by the RIP may be obtained from a colordocument scanner or a digital camera or generated by a computer or froma memory or network. The RIP may perform image processing processesincluding color correction, etc. in order to obtain the desired colorprint. Color image data is separated into the respective colors andconverted by the RIP to halftone dot image data in the respective colorusing threshold matrices, which comprise desired screen angles andscreen rulings. The RIP may be a suitably programmed computer and/orlogic devices and is adapted to employ stored or generated thresholdmatrices and templates for processing separated color image data intorendered image data in the form of halftone information suitable forprinting.

With reference now to FIG. 3 and assuming a print job request is for aphotographic type of image (as opposed to a text image) each module ofprinter apparatus 500 will be caused to generate a halftone screenedimage in the respective separation color and wherein the cells of thehalftone screen are oriented at the angle indicated in FIG. 3. Inaddition to indicating the respective angle of the screen FIG. 3 alsoidentifies with each color the respective type of dot pattern structure,which is associated with that color. For example, the yellow halftonescreen is indicated as being at 0° and a dot structure dot growthpattern is the type of dots that are created at the pixel locationswithin a halftone cell forming part of the yellow color separationhalftone screen. An example of such a dot structure dot growth patternis a full-type dot structure illustrated in FIGS. 4, 5, and 6. The cyancolor separation component halftone screen is directed at an angle of15° to that of the yellow color halftone separation screen. The cyancolor image is produced in accordance with a dot type structure similarto that used for yellow. The black color separation component halftonescreen is directed at an angle of 45° to that of the yellow colorhalftone separation screen. Note, that screen angles are nominal valuesand might vary ±0.5° from the recited number. The dot structure used toform the image in the black color is that of the line structure dotgrowth pattern type as will be more fully described with regard to FIG.7. The magenta color separation component halftone screen is directed atan angle of 75° to that of the yellow color halftone separation screen.The magenta color image is produced in accordance with a dot structuredot growth pattern type similar to that used for yellow. The blue colorseparation component halftone screen is directed at an angle of 135° tothat of the yellow color halftone separation screen. The blue colorimage is produced in accordance with a line structure dot growth patterntype similar to that used for black. It will be noted that the bluecolor separation halftone screen is directed at an angle of 90° to thatof the black color separation halftone screen. Because each of thescreens, blue and black, use the same type of growth pattern forreproducing the respective dots on the respective color separationimages that the line structures of these dots will tend to beperpendicular to each other. That is the line structures of dots in theblue color separation image will tend to be perpendicular to the linestructures of dots in the black color separation image. It has beenfound that such will greatly diminish the generation of moiré artifacts.The yellow, cyan and magenta halftone screens are at the standard anglesbut employ a dot structure dot growth pattern rather than a linestructure dot growth pattern and moiré artifacts are reduced because ofadhering to the standard angles for these respective colors. In theembodiment of FIG. 3 the line frequencies for the five screen patternsare about the same nominal screen frequency for example about 155 linesper inch. Alternatively, each may be about 133 lines per inch. Althoughit is known in the prior art to use, in a four-color tandem printer,screen patterns wherein a first of two color separation images havetheir respective halftone screen patterns oriented at 90° to each otherand a second of two color separation images that have their respectivehalftone screen patterns oriented at 90° to each other, all four of suchscreens employed a line type growth pattern. Thus where a fifth colorhalftone screen is required to be used the creation of moiré artifactswould be problematic where all halftone screen patterns employed theline structure dot growth pattern.

With reference now to FIGS. 4-6 description will be provided of a dotstructure dot growth pattern, which is distinguished herein from theline structure dot growth pattern description of which will be providedhereafter with regard to discussion of FIG. 7. It should be understoodthat while the preferred embodiments described herein utilize gray levelprintheads that are adapted to print gray level dots at each pixellocation that the invention regarding the use of various screen anglesand dot types are also suited for binary printheads that can placeeither a dot or no dot at a particular pixel location in a halftonecell. As noted above the pixel locations are grouped into cells havingcell gray levels. The dots of a cell are formed such that for eachincrease in cell gray level, a dot at, at least one of the pixels in thecell, the core pixel, forms to a larger dot size (or dot density). In anexample of one type of growth pattern, the dots are sequentially formedat the pixels in the cell in a pre-defined order such that at the lowercell gray levels a dot is formed at a first or core pixel locationwithin the cell and this dot is increased in size (or density) withdesired increases in cell density until a maximum dot size is reachedbefore beginning the formation of a dot at an adjacent pixel locationwithin the cell. Thereafter for increasing cell gray levels the dot sizeis increased at this adjacent pixel location until a maximum dot size isreached at the pixel location. Additional increases in cell density aremade similarly with buildup of dots using adjacent pixel locations sothat dot growth is from a center or core pixel location graduallyoutwardly and surrounding the central pixel location.

Alternatively, the growth pattern for the dots of the halftone cell maybe a “partial dot” dot structure dot growth pattern also described inU.S. Pat. No. 5,258,849, the contents of which are incorporated hereinby reference.

In lieu of the “full dot” growth pattern and the “partial dot” growthpattern just described as well as described in U.S. Pat. No. 5,258,849,the growth pattern may also be that which is known as a “mixed dot” dotstructure dot growth pattern wherein growth of the dot at a core pixellocation is to a predetermined level less than a maximum beforecommencing growth at one more adjacent pixel locations about the corepixel. Subsequent growth is by additions to the core pixel as well as tothe one or more adjacent pixel locations.

With reference to FIG. 4 there is illustrated an example of a3-bits/pixel gray halftone dot layout for a full dot type growthpattern. Also illustrated are seven different pixel dot sizescorresponding to the sizes that each individual pixel dot can obtain.There are 57 possible gray levels for the exemplary eight-element cell30 shown here. An example of the formation of the cell that is a graylevel 12 will now be given.

The pixel circled in level one, reference numeral 1, is formed to dotsize 1 in level 1. Only one cell will be described, although the pixelsin other cells will be changed according to the same layout or growthpattern as shown in FIG. 4. The dot at this pixel grows larger andlarger as cell gray levels increase from level 1 to level 2 all the wayto cell gray level 7. One can see that the circled pixel increases invalue from pixel size (or density) 1 to 7 as the cell gray levelsincrease. If the desired gray level for the cell 30 is 7, then theformation of dots would be completed once the circled pixel has reachedthe dot size of 7. In this example, however, the gray level for the cell30 is desired to be 12. At gray level 7, the circled pixel has reachedits maximum dot size, so that a dot at another pixel location in thecell must now start forming. This dot starts forming at an adjacentpixel indicated with a square around it in level 1, with the numeral 8.

The dot formation process continues, with the dot at the second pixelgrowing larger and larger as the levels again increase from level 1 tolevel 5. The formation process stops at level 5, since the pixel has nowreached the value of 12. The halftone cell 30 now contains, as seen inFIG. 5, a dot of dot size 7, and a dot of dot size 5. The extension ofthis formation process to 57 gray levels is easy to see from thisexample. Although this example is illustrated with a printer that formsdots in accordance with 3 bits/pixel the invention is suited for usewith any gray level printhead that can form pixels at 2 bits/pixel ormore as well as with binary printheads wherein the dot growth pattern isaround a central or core pixel location.

The full dot type process thus involves forming dots at the highestpriority pixels to their maximum allowable dot size before beginning theformation of the dots for the next highest priority pixels. An exemplaryhalftone dot mask 32 with pixel priorities indicated is shown in FIG. 6.Different matrix sizes, cell shapes, and priorities can be used for thecells than that illustrated in FIG. 4. Halftone cells of about 4×4pixels are known and may be used it being understood that the averagefor a cell need not be a whole number.

In the electrophotographic process, the full dot type formation processis favored because it forms stable dots and exhibits less granularity(halftone printing noise). The partial dot type is known to carry moreinformation detail than full dot type but at the cost of less stabledots in the electrophotographic process. The mixed dot type combines themerits of both the full dot and the partial dot types in gray levelhalftoning. The above description of the 3-bits/pixel printhead case mayreadily be extended to higher gray levels. In an example of a4-bits/pixel printhead each pixel location in the cell may have graylevels from 0 to 15. Also, in an example of a 8-bits/pixel printheadeach pixel location in the cell may have gray levels from 0 to 255 andthe resolution of the printer for printing pixels may be 300 dpi orgreater, the example of FIG. 2 being a 600 dpi printer.

With reference now to FIG. 7 and with further reference to U.S. Pat. No.5,258,850 there is shown a halftone cell wherein the growth pattern ofthe dots within the cell is development or creation along a line orlines as opposed to growing of dots about a central or core dot. In theline structure dot growth pattern for lower values of cell density, forexample from cell density level 1 to cell density 5, all the pixellocations indicated as having a dot are exposed to form a line of dotsare a series of dot lines (three dot lines are shown in the FIG. 7. Asthe lines become solid dot lines due to overlap of adjacent dots furtherincreases in cell density are produced by forming secondary linesadjacent each of the solid lines as shown in the FIG. 7. Thus it can beseen that the line structure dot growth pattern emphasizes creation ofstable line structures as opposed to the full or partial dot or mixedtype growth structure, which emphasizes stable dot growth.

With reference now to FIG. 8 there is illustrated a second embodiment ofthe invention. In this embodiment the blue, black and yellow halftonescreens feature a dot structure dot growth pattern with the screenangles being respectively 75°, 45°, and 0°. The magenta and cyanhalftone screens are situated perpendicular to each other with the cyanhalftone screen being at 15° and the magenta halftone screen being at105°. Both the magenta and cyan halftone screens feature a linestructure dot growth pattern within each cell and thus the patterns ofthe lines created within the halftone cells of the magenta screen willbe perpendicular to the lines created within the halftone cells of thecyan screen. Therefore, moiré artifacts also tend to be reduced and lessapparent for this screen system because the halftone screens employingthe dot structure dot growth pattern are between 30° and 45° apart andthe halftone screens employing the line structure dot growth pattern are90° apart.

With reference now to FIG. 9 there is illustrated a third embodiment ofthe invention. In this embodiment there are only four primary colorscyan, magenta, yellow and black and each has an associated halftonescreen. The yellow, cyan, black and magenta halftone screens aresituated at the known angles respectively of 0° (or 45°), 15°, 45° and75°. This embodiment differs from the prior art in that some of thehalftone screens, for example yellow and black, are formed with a dotstructure dot growth pattern and others, such as for example cyan andmagenta, feature a line structure dot growth pattern. In this examplethe cyan and magenta halftone screens are provided with line structurescreens so that the two directions of these lines are separated by a 60°angle and form a “double-line” screen, the white holes of which have adiamond shape, see in this regard the diamond screen pattern shown inFIG. 10. In FIG. 11 there is illustrated a density ramp of the combinedcyan and magenta screens with the diamond structure. The halftone screensystem of FIG. 9 lends itself to extension to a hi-fi five-colorprinting while minimizing unpleasant moiré patterns.

With reference now to FIGS. 12A and 12B there are illustrated fourth andfifth embodiments of the invention wherein five colors are accommodated.In each of these embodiments four of the colors, magenta, blue, cyan andblack, each feature a line structure dot growth pattern in theirrespective halftone cells as illustrated, for example, in the FIG. 12Athe screen angles are at approximately 22.5°, 67.5°, −22.5°, and −67.5°(more generally, 45° apart), the 5th screen, the yellow screen, being adot screen at 45° with frequency 1.31 times that of the line screens,and for the embodiment of FIG. 12B the screen angles are at 8.13°, 45°,98.13°, and 135°, the 5th screen, the yellow screen, being a dot screenat 71.57° with frequency 1.12 times that of the line screens. The yellowhalftone cell is oriented at an angle midway between two of the halftonecells having the screens with the line structure dot growth pattern. Forexample as indicated in FIG. 12A the angle of the yellow halftone screenis midway between that of the angles of the magenta and blue halftonescreens. In addition the yellow halftone screen features a dot structuredot growth pattern and a line frequency of at least and preferably morethan 1.1 times the line frequency of the lowest halftone screen. It willbe understood that even where the dot structure dot growth pattern isemployed that the dots in each of the cells tend to line up along linesin accordance with the screen angle of the respective screen for thatcolor. It will be further understood that the term “midway” impliesbeing within plus or minus 1° from the actual midpoint. Typical screenfrequencies range from 130 lines per inch (lpi) to 220 lpi.

With reference now to FIG. 13 there is illustrated a sixth embodiment ofthe invention wherein five colors are accommodated. In this embodimentfor the colors, cyan, black, magenta and blue, each feature a linestructure dot growth pattern in their respective halftone cells and thehalftone cells are of angles 15°, 45°, 105°, and 135° respectively. Itwill be noted that the line structures formed in the blue halftone cellswill be perpendicular to that of the line structures formed in the blackhalftone cells. Furthermore the line structures formed in the magentahalftone cells will be perpendicular to the line structures formed inthe cyan halftone cells. Therefore, objectionable moiré patternartifacts will tend to be minimized. The yellow color is formed usinghalftone cells at a 90° degree angle and using a dot structure dotgrowth pattern. Since yellow is not a dominant color it tends not to bea major contributor to objectionable moiré artifact generation.

With reference now to FIG. 14 there is illustrated a seventh embodimentof the invention wherein five colors are accommodated. The halftonescreens for the yellow and black separations are situated at 0° and 45°respectively and each of these screens employs a dot structure dotgrowth pattern. The cyan and magenta color separation images have theirrespective halftone screens at angles of 15° and 75° respectively andeach of these halftone screens employs a line structure dot growthpattern. A 60° line structure is thus formed by the cyan and magentacolor separation images, and this 60° rosette, or diamond line structureis relatively pleasing. The blue color separation image is formed usingtwo halftone screens, one being situated at 105° and the other at 165°to provide a 60° separation between the 2 screens. Both of these bluecolor separation halftone screens also employ a line structure.

With reference to the flowchart of FIG. 16 the actual image that isprinted for the blue color separation is a composite sum ofcorresponding pixel locations in each of these two blue color separationscreens. Thus as can be seen in the flowchart of FIG. 16 color imagedata of the blue color separation image is input from a color scanner,digital camera, memory or computer or generated from some othercombination of colors and typically is continuous image information andmay be subject to color correction and other corrections to make theimage data color dependent on the characteristics of the printer, step100. The corrected blue color separation image data is processed by thescreen generator at each of two different halftone screen angles, steps101, 102. To do this, threshold values are assigned with each screen andassociated with each halftone cell and dependent upon line frequency.The incoming blue image data is compared with the threshold values todetermine whether or not a dot is to be printed at a particular pixellocation i,j (binary printing case) or to determine the gray level ofthe dot at a particular pixel location i,j (gray level printing case).The algorithmically developed gray value for each pixel location i,j isthen multiplied by a predetermined weighting value, steps 103, 104, andthen the weighted products are summed in step 105. The sum in step 105represents the rendered pixel value to be sent to the printer forprinting by that color module at the pixel location i,j on the receiversheet, step 106. Further modification of the pixel value may be made foruniformity correction or otherwise as is well known in the art. Theresulting image produced has an influence of both screens so that thereare series of dots or line structures that appear to be printed alonglines at one screen angle and other series of dots or line structuresthat appear to be printed along lines at the second screen angle.Alternatively, the single color separation merge may be separatelyprocessed at the different screen angles and printed separately on thereceiver sheet to form the combined diamond structure or rosettestructure in the single color.

Halftone cells comprising the halftone screens form, in response to theimage data, a buildup of dots at various locations on each halftonescreen wherein the dots appear to be arranged along lines havingdifferent respective angles which would be at 60° to each other becausethe halftone screens are developed at a 60° angle to each other. It willbe noted that while each cell comprises plural pixel locations that itis the cell itself that is to be representative of the gray level to beprinted at an area on the receiver member. Each halftone screen 101, 102has a counterpart pixel location that would ordinarily be used to printa pixel at a pixel location i,j on the receiver. The counterpart pixelin each halftone screen is multiplied by a weighting factor associatedwith each screen and then the sum is taken and sent to the printer forprinting at that pixel location i,j on the receiver member. Thecomposite image thus formed for this blue color separation representspixels arranged along two line directions that in effect present arosette or diamond grid having a similar diamond structure to that shownwith regard to the density ramp of FIG. 11 but being in only the onecolor blue.

It has been found that this 60° diamond grid has a relatively pleasingappearance. The weights provided when forming the composite of thehalftone screens 101, 102 will typically be 0.5 for each in the case ofhalftone screens each having a line structure dot growth pattern as maybe seen in FIG. 17 and preferably each of these screens will be of thesame screen frequency. However, this weighting factor may be adjusted inaccordance with providing more weight to one than the other to emphasizeone screen angle or screen frequency over the other, see the exampleillustrated in FIG. 18. For line structure dot growth pattern screensthe weighting factor for each screen should be in the range of 0.4 to0.6 wherein the sum of the weighting factors equals 1.0. While theprocessing of the blue color or hi-fi color separation image ispreferred using two halftone screens at the aforesaid 60° angle betweenthem and processing the color separation image using a line structuredot growth pattern for each of the two halftone screens, a range ofsuitable angles between the respective two halftone screens forprocessing a color separation image to form a diamond structure is 53°to 64°.

With reference to FIG. 15 there is illustrated an eighth embodiment ofthe invention wherein five colors are accommodated. The halftone screensof yellow and black are both situated at 45° and both of these screensemploy a dot structure dot growth pattern. The cyan and magenta halftonescreens are at 18° and 72° respectively and both of these screens employa line structure dot growth pattern. The blue color separation image isformed using a composite of two halftone screens, one at −18° and theother at −72°, and in accordance with the process illustrated by theflowchart of FIG. 16 each using the line structure dot growth pattern toform the diamond line structure illustrated by FIGS. 10 and 11. Althoughthe flowchart of FIG. 16 has been illustrated for use with a linestructure dot growth pattern type of processing for a color separationimage, it is also possible, as noted with more particularity in thecross-referenced application, to employ this process for use with a dotstructure dot growth pattern type of processing for the color separationimage.

There has thus been shown an improved printer and method of printing andmethod of encoding image data wherein color images may be printed withminimization of artifacts through representation of certain colorseparation images in a line structure dot growth pattern format within ahalftone cell while color separation images of other colors arerepresented using a dot structure dot growth pattern within theirrespective halftone cells. In its broader aspects the inventioncontemplates that at least three halftone screen patterns for printingof at least three different colors may be provided for wherein two ofthe halftone screen patterns employ a similar pattern of growth of dotgrowth as either the growth of dots in accordance with a line structuredot growth pattern or the growth of dots in accordance with a dotstructure dot growth pattern and wherein the third halftone screenpattern employs a pattern of dot growth different from that of the twohalftone screen patterns. The dots formed by the various halftone screenpatterns and placed on the receiver may be superimposed on each other atthe same pixel location to form various shades of other colors.

The invention has been described with reference to the preferredembodiments. Obviously, modifications and alternatives will occur toothers upon reading in understanding the preceding detailed description.For example, as noted above that while the creation of gray level dotsin individual pixel locations has been described as the preferredembodiment the invention in its broader aspects also contemplates theuse of binary pixels for forming the line structure dot growth patternsin a halftone cell and/or the dot structure dot growth patterns in ahalftone cell. It is intended therefore that the invention be construedas including all such modifications and alternatives so far as they comewithin the scope of the appended claims or the equivalents thereof.

1. An apparatus for configuring image information for printing of animage having at least three different colors, comprising: a screengenerator or generators for generating halftone screen color separationimage data for each of the at least three different colors, the screengenerator or generators generating image data in the form of halftonescreens representative of the image at predetermined respective screenangles wherein for each of at least two of the three colors a first typeof dot growth is pattern is provided and is operational to generatehalftone cells of different density and for the third of the threecolors a second type of dot growth pattern is provided which isdifferent from the first type of dot growth pattern used commonly forthe said at least two of the three colors, wherein the first type ofgrowth pattern is selected from the group consisting of the dotstructure dot growth pattern and the line structure dot growth patternand the second type of growth pattern is selected from the groupconsisting of the dot structure dot growth pattern and the linestructure dot growth pattern and further wherein the respective screenangles for the at least two of the three colors and for the third of thethree colors are all different from each other.
 2. The apparatus ofclaim 1 and wherein the at least two of the three colors are generatedwith a line structure dot growth pattern and the third of the at leastthree colors is generated with a dot structure dot growth pattern. 3.The apparatus of claim 2 and wherein the at least three different colorsare at least five colors and the at least five colors include cyan,magenta and yellow and at least four of the at least five colors areformed with halftone cells of different screen angles.
 4. The apparatusof claim 3 and wherein the at least five colors includes blue.
 5. Theapparatus of claim 4 and wherein the dot structure dot growth patternand the line structure dot growth pattern employ the creation ofdifferent gray levels at pixel locations.
 6. The apparatus of claim 5and wherein one of the at least five colors includes black and the angleof the black halftone screen is directed at an angle that isperpendicular to the blue halftone screen.
 7. The apparatus of claim 3and wherein the line structures formed by the halftone cells with theline structure dot growth pattern are perpendicular to each other. 8.The apparatus of claim 3 and wherein the line structures formed by thehalftone cells with the line structure dot growth pattern are at anangle of between 53° and 64° to each other.
 9. The apparatus of claim 8and wherein the yellow color is formed with a halftone screen generatedprinter dot structure dot growth pattern and at a line frequency of atleast 1.1 times the line frequency of the lowest line frequency employedfor any of the other colors.
 10. The apparatus of claim 9 and whereinthe yellow halftone screen is at an angle ±10 of midway between therespective angles of two halftone screens of different colors.
 11. Theapparatus of claim 1 and wherein one color is formed as a composite oftwo halftone screens of different screen angles.
 12. The apparatus ofclaim 11 and wherein the one color formed as a composite of two halftonescreens is a composite of two halftone screens at a 60° angle to eachother.
 13. The apparatus of claim 1 in combination with a printer forprinting the image information as a halftone image in the at least threecolors.
 14. A method for configuring image information for printing ofan image having at least three different colors, comprising: generatinghalftone screen color separation image data for each of the at leastthree different colors in the form of halftone screens representative ofthe image at predetermined respective screen angles wherein for each ofat least two of the three colors a first type of dot growth is patternis provided and is operational to generate halftone cells of differentdensity and for the third of the three colors a second type of dotgrowth pattern is provided which is different from the first type of dotgrowth pattern used commonly for the said at least two of the threecolors, wherein the first type of growth pattern is selected from thegroup consisting of the dot structure dot growth pattern and the linestructure dot growth pattern and the second type of growth pattern isselected from the group consisting of the dot structure dot growthpattern and the line structure dot growth pattern and further whereinthe respective screen angles for the at least two of the three colorsand for the third of the three colors are all different from each other.15. The method of claim 14 and wherein the at least two of the threecolors are generated with a line structure dot growth pattern and thethird of the at least three colors is generated with a dot structure dotgrowth pattern.
 16. The method of claim 14 and wherein the at leastthree different colors are at least five colors and the at least fivecolors include cyan, magenta and yellow and at least four of the atleast five colors are formed with halftone cells of different screenangles.
 17. The method of claim 16 and wherein the at least five colorsincludes blue.
 18. The method of claim 17 and wherein the dot structuredot growth pattern and the line structure dot growth pattern employ thecreation of different gray levels at pixel locations.
 19. The method ofclaim 18 and wherein one of the at least five colors includes black andthe angle of the black halftone screen is directed at an angle that isperpendicular to the blue halftone screen.
 20. The method of claim 14and wherein the line structures formed by the halftone cells with theline structure dot growth pattern are perpendicular to each other. 21.The method of claim 14 and wherein the line structures formed by thehalftone cells with the line structure dot growth pattern are at anangle of between 53° and 64° to each other.
 22. The method of claim 21and wherein yellow is one of the at least five colors and the yellowcolor is formed with a halftone screen generated printer dot structuredot growth pattern and at a line frequency of at least 1.1 times theline frequency of the lowest line frequency employed for any of theother colors.
 23. The method of claim 22 and wherein the line structureof the yellow halftone screen is at an angle ±1° of midway between therespective angles of two halftone screens of different colors.
 24. Themethod of claim 14 and wherein one color is formed as a composite of twohalftone screens.
 25. The method of claim 14 and wherein the one colorformed as a composite of two halftone screens is a composite of twohalftone screens at a 60° angle to each other.
 26. The method forgenerating a multicolor image using halftone screens comprising: forminghalftone dots using a dot structure dot growth pattern for the halftonecells of one or more of the colors; and forming halftone lines using aline structure dot growth pattern for the halftone cells of at least twoof the other colors.